Printing media are improved if they resist curl and remain flat. This is particularly important with media utilized with the formation with color images. Such media include those utilized for ink jet, thermal dye sublimation imaging, thermal wax dye transfer, and electrophotography. The formation of a flat base is particularly important for color photographic paper.
In the formation of color paper it is known that the base paper has applied thereto a layer of polymer, typically polyethylene. This layer serves to provide waterproofing to the paper, as well as providing a smooth surface on which the photosensitive layers are formed. The formation of a suitably smooth surface is difficult requiring great care and expense to ensure proper laydown and cooling of the polyethylene layers. One defect in prior formation techniques is caused when an air bubble is trapped between the forming roller and the polyethylene which will form the surface for casting of photosensitive materials. This air bubble will form a pit that will cause a defect in the photographic performance of photographic materials formed on the polyethylene. It would be desirable if a more reliable and improved surface could be formed at less expense.
In color papers there is a need for providing color papers with improved resistance to curl. Present color papers will curl during development and storage. Such curl is thought to be caused by the different properties of the layers of the color paper as it is subjected to the developing and drying processes. Humidity changes during storage of color photographs lead to curling. There are particular problems with color papers when they are subjected to extended high humidity storage such as at greater than 50% relative humidity. Extremely low humidity of less than 20% relative humidity also will cause photographic papers to curl.
In photographic papers the polyethylene layer also serves as a carrier layer for titanium dioxide and other whitener materials as well as tint materials. It would be desirable if the colorant materials rather than being dispersed throughout the polyethylene layer could be concentrated nearer the surface of the layer where they would be more effective photographically.
It has been proposed in U.S. Pat. No. 5,244,861 to utilize biaxially oriented polypropylene in receiver sheets for thermal dye transfer.
There remains a need for photographic substrates that will resist bending forces. These bending forces result in curled photographs which are undesirable as they may not be stacked well, displayed on flat surfaces, or displayed in a pleasing manner in albums or when viewed individually.